Cold rolled steel sheet and hot rolled steel sheet excellent in bake hardenability and resistance to ordinary temperature aging and method for their production

ABSTRACT

To provide a steel sheet excellent in painting bake hardenability and anti aging property at room temperature: containing, in mass, 0.0001 to 0.20% of C, 2.0% or less of Si, 3.0% or less of Mn, 0.15% or less of P, 0.015% or less of S, and, in addition, 010% or less of Al and 0.001 to 0.10% of N so as to satisfy the expression 0.52Al/N&lt;5 and, further, one or more of 2.5% or less of Cr, 1.0% or less of Mo and 0.1% or less of V so as to satisfy the expression (Cr+3.5MO+39V) ≧0.1, with the balance consisting of Fe and unavoidable impurities; having the value of BE170, evaluated after applying a 2% tensile deformation and then a heat treatment at 170° C. for 20 min., being 45 MPa or more, and any of the value of BH160, evaluated after applying a 2% tensile deformation and then a heat treatment at 160° C. for 10 min., and the value of BH150, evaluated after applying a 2% tensile deformation and then a heat treatment at 150° C. for 10 min., being 35 MPa or more; and having the yield point elongation at a tensile test after applying a heat treatment at 100° C. for 1 h. being 0.6% or less.

TECHNICAL FIELD

[0001] This invention relates to a steel sheet having painting bakehardenability (BH), anti aging property at room temperature andformability at the same time, and a method of producing the steel sheet.

[0002] The letters BH are an abbreviation of bake hardenability or bakehardening and it means a simplified evaluation, by means of a tensiletest, of the increase in the mechanical strength of a steel sheetresulting from the baking of a painting after press forming in carmanufacturing. BH is measured as follows: first, the flow stress of asteel sheet is measured under a 2% tensile deformation imposed at atensile test; then, after a prescribed heat treatment (usually, at 170°C. for 20 min., but heat treatments at 150° C. and 160° C. are alsoincluded in the present invention), the upper yield stress of the steelsheet is measured in another tensile test; suppose the flow stress atthe first tensile test under the 2% tensile deformation is σ1 and theupper yield stress at the second tensile test is σ2, the amount of EH isgiven as σ2-σ1. Note that, when there is no upper yield point, the 0.2%proof stress of the steel sheet is used.

[0003] A steel sheet according to the present invention is used forcars, home electric appliances, buildings, etc. and it includes both acold-rolled or hot-rolled steel sheet in the narrow sense of the wordwithout surface treatment and a cold-rolled or hot-rolled steel sheet inthe broad sense of the word with surface treatment such as alloying hotdip galvanizing, electrolytic plating, etc. as an anti-corrosionmeasure.

BACKGROUND ART

[0004] The production of ultra low carbon steels has been made easierthanks to the latest technical advancement of the vacuum degassingtreatment of molten steel, and the demands for ultra low carbon steelshaving excellent workability has been increasing. Among this kind ofproduct, the ultra low carbon steel sheets containing Ti and Nb added incombination disclosed in Japanese Unexamined Patent Publication No.S59-31827 and the like, for example, have painting bake hardenability(BH) as well as extremely good workability, and are excellent also inhot dip galvanizing property. For this reason, these steel sheets havecome to claim a significant position in the market.

[0005] The amount of BH of the steel sheets, however, is not beyond thelevel of those of conventional BH steel sheets, and they have ashortcoming that, when it is attempted to increase the amount of BH ofthe steel sheets, it becomes impossible to maintain their anti agingproperty at room temperature.

[0006] A steel sheet having an enhanced BH is excellent in workabilitythanks to its low strength at the stage of press forming, and is alsoexcellent especially in dent resistance owing to the fact that itbecomes hard after it is finally formed into the shape of a productcomponent. Generally speaking, when the amount of solute C or solute Nin steel is increased, the amount of BH is increased but, on the otherhand, anti aging property at room temperature poses a problem.

[0007] As an example of the technology related to a steel sheet havingboth high bake hardenability and anti aging property at roomtemperature, Japanese Examined Patent Publication No. H3-2224 proposes atechnology to obtain a cold-rolled steel sheet having a high r-value,high bake hardenability, good ductility and anti aging property at roomtemperature at the same time, by adding a large amount of Nb, B and Ti,together, to an ultra low carbon steel so as to make the annealedstructure of the steel a composite structure consisting of a ferritephase and a phase formed through low temperature transformation.

[0008] It has been made clear, however, that the proposed technology hasproblems related to actual production operation as described in 1) and2) below.

[0009] 1) In a steel having a composition comprising a large amount ofNb, B and Ti, the transformation point where the steel transforms from αphase to γ phase does not fall and thus annealing at a very hightemperature is required for obtaining a composite structure, which inturn causes troubles such as strip breakage during continuous annealing.

[0010] 2) Since the temperature range where the steel has an α+γ phaseis very narrow, there arises the case that the structure varies alongthe width of the steel sheet resulting in the large dispersion ofproduct quality or that the structure may or may not become a compositestructure depending on the fluctuation of annealing temperature byseveral degrees Celsius. Therefore, the production tends to be veryunstable.

[0011] As another example, Japanese Unexamined Patent Publication No.H7-300623 teaches that it is possible to obtain both a high BE value andanti aging property at room temperature, by increasing the carbonconcentration at crystal grain boundaries of an ultra low carboncold-rolled steel sheet containing Nb through controlling the coolingrate after annealing. The technology disclosed therein, however, doesnot realize a high BH value and anti aging property at room temperaturein a sufficiently well-balanced manner.

[0012] There is another problem in conventional BH steel sheets that,whereas a prescribed amount of BH is obtained as far as the heattreatment of BH is conducted under a condition of 170° C. for 20 min.,the amount of BH is lowered under a heat treatment condition of 160° C.for 10 min. or 150° C. for 10 min.

[0013] As described above, conventional BH steel sheets haveshortcomings that stable production is difficult and that anti agingproperty at room temperature is lost when the amount of BH is increasedFurther, they have another problem in that a sufficient amount of BH isnot obtained when the temperature at the baking of a painting is loweredfrom currently adopted 170° C. to 160 or 150° C.

DISCLOSURE OF THE INVENTION

[0014] The object of the present invention is to provide a steel sheethaving both high bake hardenability and anti aging property at roomtemperature and capable of maintaining a sufficient amount of BH evenunder a low BH temperature, and a method of producing the steel sheet.

[0015] As a result of assiduous studies for achieving the above object,the present inventors obtained the following finding which was hithertounknown.

[0016] That is to say, the present inventors discovered that it waspossible, by adding Cr, MO, V and so forth to a steel retaining soluteN, to obtain both a high BH value and anti aging property at roomtemperature and maintain high bake hardenability even when the baking ofa painting was conducted at a lower temperature for a shorter period oftime.

[0017] The present invention is a totally new steel sheet which washitherto unknown to the market, worked out on the basis of thephilosophy and findings described above, and a method of producing thesteel sheet. The gist, therefore, is as follows:

[0018] (1) A cold-rolled steel sheet excellent in painting bakehardenability and anti aging property at room temperature characterizedby: containing, in mass, 0.0001 to 0.20% of C, 2.0% or less of Si, 3.0%or less of Mn, 0.15% or less of P, 0.015% or less of S and, in addition,0.10% or less of Al and 0.001 to 0.10% of N so as to satisfy theexpression 0.52Al/N<5 and, further, one or more of 2.5% or less of Cr,1.0% or less of Mo and 0.1% or less of V so as to satisfy the expression(Cr+3.5Mo +39V)≧0.1, with the balance consisting of Fe and unavoidableimpurities; having the value of BE170, which is evaluated after applyinga 2% tensile deformation and then a heat treatment at 170° C. for 20min., equal to or more than 45 MPa, and any of the value of BH160, whichis evaluated after applying a 2% tensile deformation and then a heattreatment at 160° C. for 10 min., and the value of BH150, which isevaluated after applying a 2% tensile deformation and then a heattreatment at 150° C. for 10 min., equal to or more than 35 MPa; andhaving the yield point elongation at a tensile test after applying aheat treatment at 100° C. for 1 h. equal to or less than 0.6%.

[0019] (2) A hot-rolled steel sheet excellent in painting bakehardenability and anti aging property at room temperature characterizedby: containing, in mass, 0.0001 to 0.20% of C, 2.0% or less of Si, 3.0%or less of Mn, 0.15% or less of P, 0.015% or less of S and, in addition,0.20% or less of Al and 0.001 to 0.10% of N so as to satisfy theexpression 0.52Al/N<10 and, further, one or more of 2.5% or less of Cr,1.0% or less of MO and 0.1% or less of v so as to satisfy the expression(Cr+3.5Mo +39v)≧0.1, with the balance consisting of Fe and unavoidableimpurities; having the value of BH170, which is evaluated after applyinga 2% tensile deformation and then a heat treatment at 170° C. for 20min., equal to or more than 45 MPa, and any of the value of BH160, whichis evaluated after applying a 2% tensile deformation and then a heattreatment at 160° C. for 10 min., and the value of BH150, which isevaluated after applying a 2% tensile deformation and then a heattreatment at 150° C. for 10 min., equal to or more than 35 MPa; andhaving the yield point elongation at a tensile test after applying aheat treatment at 100° C. for 1 h. equal to or less than 0.6%.

[0020] (3) A cold-rolled or hot-rolled steel sheet excellent in paintingbake hardenability and anti aging property at room temperature accordingto the item (1) or (2), characterized by containing 0.0005 to 0.004%, inmass, of solute N.

[0021] (4) A cold-rolled or hot-rolled steel sheet excellent in paintingbake hardenability and anti aging property at room temperature accordingto any one of the items (1) to (3), characterized by containing,further, 0.0005 to 0.01%, in mass, of Ca.

[0022] (5) A cold-rolled or hot-rolled steel sheet excellent in paintingbake hardenability and anti aging property at room temperature accordingto any one of the items (1) to (4), characterized by containing, yetfurther, 0.0001 to 0.001%, in mass, of B.

[0023] (6) A cold-rolled or hot-rolled steel sheet excellent in paintingbake hardenability and anti aging property at room temperature accordingto any one of the items (1) to (5), characterized by containing, inaddition, 0.001 to 0.03%, in mass, of Nb.

[0024] (7) A cold-rolled or hot-rolled steel sheet excellent in paintingbake hardenability and anti aging property at room temperature accordingto any one of the items (1) to (6), characterized by containing,moreover, 0.0001 to 0.10%, in mass, of Ti so as to satisfy theexpression (N−0.29Ti)>0.0005.

[0025] (8) A cold-rolled or hot-rolled steel sheet excellent in paintingbake hardenability and anti aging property at room temperature accordingto any one of the items (1) to (7), characterized by containing,furthermore, one or more of Sn, Cu, Ni, Co, Zn, W, Zr and Mg to a totalof 0.001 to 1.0%, in mass.

[0026] (9) A galvanized cold-rolled steel sheet excellent in paintingbake hardenability and anti aging property at room temperaturecharacterized by applying a hot dip galvanizing, an alloying hot dipgalvanizing or an electrogalvanizing to a cold-rolled steel sheetaccording to any one of the items (1) and (3) to (8).

[0027] (10) A galvanized hot-rolled steel sheet excellent in paintingbake hardenability and anti aging property at room temperaturecharacterized by applying a hot dip galvanizing, an alloying hot dipgalvanizing or an electrogalvanizing to a hot-rolled steel sheetaccording to any one of the items (2) and (3) to (8).

[0028] (11) A method of producing a cold-rolled steel sheet excellent inpainting bake hardenability and anti aging property at room temperaturecharacterized by: hot-rolling a slab having the chemical compositionaccording to any one of the items (1) and (3) to (8) at a temperature100° C. below the Ar₃ transformation temperature or higher; cold-rollingthe hot-rolled steel sheet thus produced at a reduction ratio of 95% orless; annealing the cold-rolled steel sheet thus produced so that themaximum heating temperature attains the temperature range from 600 to1,100° C.; and then cooling it from the annealing temperature to atemperature of 400° C. or lower at an average cooling rate of 10°C./sec. or more.

[0029] (12) A method of producing a cold-rolled steel sheet excellent inpainting bake hardenability and anti aging property at room temperaturecharacterized by: hot-rolling a slab having the chemical compositionaccording to any one of the items (1) and (3) to (8) at a temperature100° C. below the Ar₃ transformation temperature or higher; cold-rollingthe hot-rolled steel sheet thus produced at a reduction ratio of 95% orless; annealing the cold-rolled steel sheet thus produced so that themaximum heating temperature attains the temperature range from 600 to1,100° C.; then cooling it from the annealing temperature to atemperature of 400° C. or lower at an average cooling rate of 10°C./sec. or more; and then applying to it an overaging treatment at thetemperature range from 150 to 400° C. for 120 sec. or longer.

[0030] (13) A method of producing a hot-dip-galvanized cold-rolled steelsheet excellent in painting bake hardenability and anti aging propertyat room temperature characterized by; hot-rolling a slab having thechemical composition according to any one of the items (1) and (3) to(8) at a temperature 100° C. below the Ar₃ transformation temperature orhigher; cold-rolling the hot-rolled steel sheet thus produced at areduction ratio of 95% or less; and then, in a continuous hot dipgalvanizing line, annealing the cold-rolled steel sheet thus produced sothat the maximum heating temperature attains the temperature range from600° C. to 1,100° C.; then cooling it from the annealing temperature tothe temperature of the galvanizing bath at an average cooling rate of10° C./sec. or more, and applying a hot dip galvanizing to it.

[0031] (14) A method of producing an alloying-hot-dip-galvanizedcold-rolled steel sheet excellent in painting bake hardenability andanti aging property at room temperature characterized by conducting aheat treatment in a temperature range from 460 to 650° C. for 3 sec. orlonger after the hot dip galvanizing specified in the method ofproducing a hot-dip-galvanized cold-rolled steel sheet according to theitem (13).

[0032] (15) A method of producing a hot-rolled steel sheet excellent inpainting bake hardenability and anti aging property at room temperaturecharacterized by: hot-rolling a slab having the chemical compositionaccording to any one of the items (2) and (3) to (8) at a temperature100° C. below the Ar₃ transformation temperature or higher; cooling thehot-rolled steel sheet thus produced from the hot rolling finishingtemperature to a temperature of 600° C. or below at an average coolingrate of 10° C./sec. or more; and then coiling it at a temperature of550° C or below.

BEST MODE FOR CARRYING OUR THE INVENTION

[0033] The reasons why the chemical composition of steel and theproduction conditions are specified as above in the present inventionwill be explained in more detail hereafter.

[0034] C is an element to increase steel strength economically, and itsaddition amount varies depending on the level of envisaged strength.However, decreasing the content of C to below 0.0001% is difficult forthe reasons of steelmaking technology, and it not only incurs a costincrease but also deteriorates the fatigue property of welded portions.For this reason, the lower limit of the addition amount of C is set at0.0001%. When the amount of C exceeds 0.20%, on the other hand,formability and weldability are adversely affected and, besides, itbecomes difficult to obtain both good bake hardenability and anti agingproperty at room temperature at the same time, which is a key issue inthe present invention. The upper limit of the addition amount of C is,therefore, set at 0.20%. When the present invention is applied to themembers to which deep drawing formability is required, it is preferableto control the content of C within a range from 0.0001 to 0.0020% orfrom 0.012 to 0.024%.

[0035] It is desirable that the amount of solute C is 0.0020% or less.Since high bake hardenability and anti aging property at roomtemperature are secured according to the present invention mainly bymeans of the addition of N, when the amount of solute C is too large, itbecomes difficult to maintain good anti aging property at roomtemperature. It is more preferable to control the amount of solute C tobelow 0.0010%. The amount of solute C may be controlled by restrictingthe amount of total C to the upper limit specified above or less,otherwise by lowering it to the prescribed level through controlling thecoiling temperature or the condition of the overaging treatment.

[0036] Si is a solid solution hardening element and increases strength.It is also effective for forming a structure containing martensite,bainite and, in addition, a retained γ phase and the like. While theaddition amount of Si varies depending on the level of envisagedstrength, when it exceeds 2.0%, press formability and a chemicaltreatment property are deteriorated. For this reason, the upper limit ofthe addition amount of Si is set at 2.0%. When an alloying hot dipgalvanizing is applied, an addition of Si in a great amount results inproblems such as low productivity caused by poorer plating adhesion andslower alloying reactions and, therefore, the upper limit of the Sicontent is set at 0.8%. No lower limit of Si is set specifically but,since lowering the Si content to 0.001% or less causes production costincrease, 0.001% is the lower limit in practical sense. If it isdifficult to deoxidize steel with Al because of a requirement to controlthe amount of Al, Si may be used for deoxidation. In this case, 0.04% ormore of Si is to be included in steel.

[0037] Mn is useful as a solid solution hardening element.

[0038] It is also effective for forming MnS to suppress the occurrenceof edge cracks caused by S during hot rolling, fining the structure ofhot-rolled sheets and forming the structure containing martensite,bainite and, in addition, a retained γ phase and the like. Moreover, Mnhas the effect to inhibit aging at room temperature caused by solute N.For these reasons, it is desirable to add 0.3% or more of Mn. When deepdrawability is required, however, it is desirable to limit the contentof Mn to 0.15% or less, preferably, to below 0.10% When the additionamount of Mn exceeds 3.0%, on the other hand, the strength becomes sohigh that ductility is decreased and the plating adhesion of galvanizingis adversely affected. The upper limit of the addition amount of Mn is,therefore, set at 3.0%.

[0039] P is known as an element to raise strength economically, like Si,thus, when it is necessary to increase strength, P is addedintentionally. P also has the effects to make fine a hot-rolledstructure and enhance workability, when it is added in excess of 0.15%,however, it deteriorates the fatigue strength after spot welding, andalso increases yield strength too much causing poor planar shape atpress forming. The excessive addition of P also lowers productivitysince it drastically slows down the alloying reactions during continuoushot dip galvanizing, and the workability in secondary working isdeteriorated, too. The upper limit of the addition of P is, therefore,set at 0.15%.

[0040] The upper limit of the addition of S is set at 0.015%, since theaddition of S in excess of 0.015% causes hot cracking and thedeterioration of workability.

[0041] Al may be added for oxidizing. However, since Al combines with Nto form AlN and, thus, lowers bake hardenability, it is desirable tolimit its addition to the least necessary amount within the range not tomake production technically difficult. From this point of view, itsupper limit for a cold-rolled steel sheet is set at 0.10% when the Alcontent exceeds 0.10%, it becomes necessary to add a large amount of Nin order to secure a required amount of solute N, which isdisadvantageous in terms of production costs and formability. A morepreferable upper limit is 0.02%, and a still more preferable upper limitis 0.007%. In the case of a hot-rolled steel sheet, on the other hand,it is possible to secure a sufficient amount of solute N by rapidcooling after hot rolling, even when more Al than N is included in termsof the number of atoms. For this reason, an upper limit of the Alcontent may be 0.20%. Production is made easier still when the Alcontent is 0.05% or less or, more preferably, 0.02% or less.

[0042] N is an important element in the present invention: good bakehardenability in the present invention is achieved mainly by using N. Itis therefore essential to add 0.001% or more of N. When the content of Nis too high, on the other hand, it becomes difficult to secure antiaging property at room temperature, or workability is deteriorated. Forthis reason, the upper limit of the N content is set at 0.10%. Apreferable range of the N content is from 0.002 to 0.020% or, morepreferably, from 0.002 to 0.008%. Besides the above, because N easilycombines with Al to form AlN, it is necessary to maintain the value of0.52Al/N equal to or smaller than a prescribed value in order to securea sufficient amount of N which contributes to the improvement of bakehardenability. In the case of a cold-rolled steel sheet, it is necessarythat the expression 0.52Al/N<5 is satisfied since AlN easilyprecipitates during the heating and the holding of the temperature in anannealing process. It is preferable to satisfy the expression 0.52Al/N<4or, more preferably, 0.52Al/N<3.

[0043] However, if annealing is conducted at a high heating rate and fora short holding time, the range of the value of 0.52Al/N may be equal tothat of a hot-rolled steel sheet.

[0044] In the case of a hot-rolled steel sheet, in contrast, the valueof 0.52Al/N is defined as follows, when the value of 0.52Al/N is 10 ormore, AlN easily precipitates during the cooling and coiling after hotrolling and, for this reason, the upper limit of the value of 0.52Al/Nhas to be below 10. When the value of 0.52Al/N is kept below 10, anexcessive precipitation of AlN can be avoided by properly controllingthe cooling rate and coiling temperature after hot rolling, and goodbake hardenability can be realized. A more preferable upper limit of thevalue of 0.52Al/N is 5.

[0045] Cr, No and V are important elements in the present invention; itis indispensable to add one or more of these elements to the steel. Goodbake hardenability and anti aging property at room temperature areobtained at the same time only when one or more of them are added.

[0046] It is known to be difficult to secure anti aging property at roomtemperature when more than a prescribed amount of N is included insteel, because N diffuses more rapidly than C does. For this reason, BHsteel sheets using N are not applied to the members for which appearanceis important such as the outer panels of a car body.

[0047] The present inventors, however, noted as a new discovery that itwas possible to obtain anti aging property at room temperature withoutdeteriorating bake hardenability, by adding Cr, Mo and/or Vintentionally.

[0048] The mechanism through which the anti aging property at roomtemperature is enhanced by one or more of these elements is notaltogether clear, but it is speculated to be as follows.

[0049] These elements form pairs and/or clusters together with N nearroom temperature, inhibiting the diffusion of N, and this secures antiaging property at room temperature. During the baking treatment ofpainting at 150 to 170° C., in contrast, N leaves of the pairs andclusters to fix dislocations, and this causes high bake hardenability toshow.

[0050] The upper limits of the addition amounts of Cr, Mo and V, whichare determined in consideration of workability and production costs, are2.5, 1.0 and 0.1%, respectively. When added too much, V forms nitridesand it becomes difficult to secure a sufficient amount of solute N.Therefore, it is desirable to limit the addition of V to 0.04% or less.

[0051] In order to secure anti aging property at room temperature, Cr,Mo and/or V must be added so that the expression (Cr+3.5Mo+39V)≧0.1 issatisfied. It is more desirable if the expression (Cr+3.5Mo+39V)≧0.4 issatisfied. Further, for securing anti aging property at roomtemperature, it is more effective to add two or more of Cr, Mo and Vtogether than to add one of them individually.

[0052] The amount of solute N has to be 0.0005 to 0.004% in total. Here,the solute N includes not only the N existing in Fe independently butalso the N forming pairs or clusters with substitutional solute elementssuch as Cr, Mo, V, Mn, Si and P. The amount of solute N can beappropriately determined by the heating extraction method in a hydrogengas flow. In the method, the amount of solute N is obtained by heating asample to a temperature range from 200 to 500° C. or so, forming ammoniathrough a reaction of the solute N with the hydrogen, analyzing theammonia thus formed by mass spectrometry, and converting the amount ofammonia thus obtained.

[0053] The amount of solute N can be calculated also by subtracting theamount of N existing as compounds such as AlN, NbN, VN, TiN, BN, etc.(determined through chemical analysis of the residue of the extraction)from the amount of total N. It may be obtained by the internal frictionmethod or the field ion microscopy (FIN), too.

[0054] When the amount of solute N is below 0.0005%, sufficient bakehardenability is not obtained. When the amount of solute N exceeds0.004%, on the other hand, while bake hardenability is improved, itbecomes difficult to obtain anti aging property at room temperature. Amore preferable range of the amount of solute N is from 0.0012 to0.003%.

[0055] Ca is effective for deoxidizing and also for controlling theshape of sulfides and, therefore, 0.0005 to 0.01% of Ca may be added.With an addition below 0.0005%, a sufficient effect is not obtained but,when added in excess of 0.01%, workability is deteriorated. For thisreason, the range of the Ca addition has to be from 0.0005 to 0.01%.

[0056] B is added, as required, by 0.0001 to 0.001% because it iseffective for preventing the embrittlement of steel during secondaryworking. With an addition below 0.0001%, a tangible effect is notobtained and, when added in excess of 0.001%, however, the effect issaturated and, besides, BN is likely to form and it becomes difficult tosecure a sufficient amount of solute N. A more preferable range of the Baddition is from 0.0001 to 0.0004%.

[0057] Nb is added, as required, within a range from 0.001 to 0.03%, asit is effective for enhancing workability and strength and also forforming a fine and homogeneous structure. When the amount of itsaddition is below 0.001%, however, the effects of its addition do notshow and, when added in excess of 0.03%, in contrast, NbN is likely toform and it becomes difficult to secure a sufficient amount of solute N.A more preferable range of the Nb addition is from 0.001 to 0.012%.

[0058] Ti has the same effects as Nb and, for this reason, it is added,as required, within a range from 0.0001 to 0.10%. When the amount of itsaddition is below 0.0001%, however, the effects do not show and, whenadded in excess of 0.10%, on the other hand, a large amount of Nprecipitates or crystallizes in the form of TiN and, thus, it becomesdifficult to secure a sufficient amount of solute N. A desirable rangeof the Ti addition is from 0.001 to 0.020% or, more preferably, from0.001 to 0.012%. Besides the above, in order to secure a sufficientamount of solute N, Ti must be added within the range to satisfy theexpression (N−0.29Ti)>0.0005 or, more preferably, (N−0.29Ti)>0.0010.

[0059] A total of 0.001 to 1.0% of one or more of Sn, Cu, Ni, Co, Zn, W,Zr and Mg may be added to a steel containing the above elements as maincomponents. However, since Zr forms ZrN, its addition is limited,desirably, to 0.01% or less.

[0060] Next, the reasons why the production conditions are specified inthe present invention will be explained.

[0061] The slab to be hot-rolled is not restricted specifically in termsof its production conditions: it may be a continuously cast slab or aslab produced using a thin slab caster or the like. A slab produced by aprocess such as the continuous casting-direct rolling (CC-DR) process inwhich the slab is hot-rolled immediately after it is cast is alsosuitable for the present invention.

[0062] In the case that a hot-rolled steel sheet is used as a finalproduct, it is necessary to specify its production conditions asfollows. The finishing temperature of the hot rolling must not be belowthe Ar transformation temperature by 100° C. or more. If the finishingtemperature is below the Ar₃ transformation temperature by more than100° C., it becomes difficult to obtain good workability or thicknessaccuracy. A more preferable finishing temperature range is the Ar₃transformation temperature or higher. No upper limit is set specificallyas to the finishing temperature of the hot rolling, but it is desirablethat the temperature is 1,100° C. or lower in order to prevent coarsecrystal grains from forming and to protect the hot rolling rolls.

[0063] Note that the heating temperature of the hot rolling is notspecifically restricted. But, when it is necessary to melt AlN in orderto obtain a sufficient amount of solute N, it is desirable to heat aslab to 1,200° C. or higher.

[0064] After hot rolling, it is necessary to cool a hot rolled steelsheet so that an average cooling rate of 10° C./sec. or more ismaintained from the finishing temperature of the hot rolling to at least600° C., in order to suppress the precipitation of AlN.

[0065] The present inventors also discovered that, even when anexcessive amount of N was added in proportion to Al, that is, even whenthe expression 0.52Al/N<1 was true, it was essential, for securing highbake hardenability and anti aging property at room temperature, to keepthe cooling rate at 10° C./sec. or higher. It is more desirable for bakehardenability and anti aging property at room temperature if the coolingrate is 30° C./sec. or higher. No upper limit of the cooling rate is setspecifically, but it is desirable from the productivity viewpoint tocool the steel sheet at a cooling rate of 200° C./sec. or lower.

[0066] In order to suppress the precipitation of AlN, the coilingtemperature has to be 550° C. or lower or, more desirably, 450° C. orlower.

[0067] The structure of the hot-rolled steel sheet obtained according tothe present invention contains ferrite or bainite as the main phase, butit is acceptable if both of them exist as a mixture. It is alsoacceptable if martensite, austenite, carbides and/or nitrides exist inthe mixture. This means that different structures may be formed inaccordance with required characteristics.

[0068] It is acceptable to apply to the steel sheet after the hotrolling, as required, a pickling and then a skin-pass rolling, eitherin-line or off-line, at a reduction ratio of 10% or less or a coldrolling at a reduction ratio up to 40% or so.

[0069] Then, the production conditions in the case that a cold-rolledsteel sheet is used as a final product are explained.

[0070] In order to obtain good workability of the final product, it isnecessary that the finishing temperature of the hot rolling is 100° C.below the Ar₃ transformation temperature or higher. No upper limit isset specifically as to the finishing temperature of the hot rolling, butit is desirable that the temperature is 1,100° C. or lower in order toprevent coarse crystal grains from forming and protect hot rollingrolls.

[0071] The reduction ratio of the cold rolling must be 95% or less. Areduction ratio exceeding 95% is undesirable because not only does theload on a rolling apparatus become too large, but also the mechanicalproperty of the product becomes largely anisotropic A desirablereduction ratio is 86% or less. No lower limit is set specifically as tothe reduction ratio of the cold rolling, but it is desirable to set thereduction ratio at 60% or more when good deep drawability is required.

[0072] The maximum temperature of the annealing must be 600 to 1,100° C.When the annealing temperature is below 600° C., recrystallization isincomplete and workability becomes poor. when the annealing temperatureexceeds 1,100° C., on the other hand, the structure becomes coarse andworkability is deteriorated. A more preferable range of the annealingtemperature is from 650 to 900° C.

[0073] The cooling after annealing is important in the presentinvention: a steel sheet having both high bake hardenability and antiaging property at room temperature can be produced only when an averagecooling rate of 10° C./sec. or higher is maintained during the coolingdown to 400° C. or lower after completing the annealing. It is desirableto set the cooling rate at 30° C./sec. or higher or, more preferably,50° C./sec. or higher. No upper limit is set specifically as to theaverage cooling rate after completing the annealing, but it ispreferable from the productivity viewpoint to conduct the cooling at200° C./sec. or lower.

[0074] The averaging treatment after the cooling may be conducted asappropriate in accordance with the objects such as the control of thestructure, the decrease of the amount of solute C, and so forth. Forobtaining both high bake hardenability and anti aging property at roomtemperature, however, it is desirable to set the averaging temperatureat 400° C. or lower, preferably 350° C. or lower or, more preferably,300° C. or lower. When the overaging treatment has to be applied, it isdesirable that its duration is 60 sec or more but, from the viewpoint ofproductivity, 600 sec. or less.

[0075] When a hot dip galvanizing is to be applied, the average coolingrate from the annealing temperature to the temperature of thegalvanizing bath has to be 10° C./sec. or higher. In this case, too, forfurther enhancing bake hardenability and anti aging property at roomtemperature, it is desirable to set the average cooling rate at 30°C./sec. or higher or, more preferably, 50° C./sec. or higher. No upperlimit is set specifically as to the average cooling rate until thegalvanizing bath, but it is preferable from the productivity viewpointto cool at 200° C./sec. or slower. When a Zn-Fe alloying treatment isrequired after the galvanizing, the steel sheet has to be reheated to460 to 650° C. and held at the temperature for 3 sec. or more or,preferably, to 470 to 550° C. and held there for 15 sec. or more. Noupper limit is set specifically for the duration of the alloying heattreatment, but it is preferable from the productivity viewpoint to limitthe time to 1 min. or less.

[0076] For further improving anti aging property at room temperature andcorrecting the shape of the steel sheet, it is desirable to apply askin-pass rolling at a reduction ratio of 2% or less. When the reductionratio exceeds 3%, yield strength is raised and the load on a rollingfacility becomes too large. The upper limit of the reduction ratio is,therefore, set at 3%.

[0077] The structure of the cold-rolled steel sheet obtained accordingto the present invention contains ferrite or bainite as the main phase,but it is acceptable if both of them exist as a mixture. It is alsoacceptable if martensite, austenite, carbides and/or nitrides exist inthe mixture. This means that different structures may be formed inaccordance with required characteristics.

[0078] The value of BH170 of the steel sheet produced according to thepresent invention is 45 MPa or higher, and any of its BH160 and BH150values is 35 MPa or higher. More preferable ranges are 60 MPa or higherfor BH170 and 50 MPa or higher for both BH160 and BH150. No upper limitsare set specifically for these values but, when the value of BH170exceeds 140 MPa and those of BH160 and BH150 exceed 130 MPa, it becomesdifficult to secure anti aging property at room temperature.

[0079] It has to be noted that; BH170 means the value of bakehardenability evaluated after applying a 2% tensile deformation and thena heat treatment at 170° C. for 20 min.; BH160 the value of bakehardenability evaluated after applying a 2% tensile deformation and thena heat treatment at 160° C. for 10 min.; and BH150 the value of bakehardenability evaluated after applying a 2% tensile deformation and thena heat treatment at 150° C. for 10 min.

[0080] The anti aging property at room temperature is evaluated in termsof the yield point elongation after an artificial aging treatment. Theyield point elongation of the steel sheet produced according to thepresent invention at a tensile test after a heat treatment at 100° C.for 1 h. is 0.6% or less. A preferable value is 0.4% or less or, morepreferably, 0.3% or less. It is desirable that the yield pointelongation after a heat treatment at 40° C. for 70 days is 0.5% or less,preferably 0.3% or less or, more preferably, 0.2% or less.

[0081] The present invention will be explained hereafter based onexamples.

EXAMPLE Example 1

[0082] Steels having the chemical compositions shown in Table 1 wereproduced and hot-rolled under the conditions shown in Table 2, whereinthe slab heating temperature was 1,250° C. for all the steels. Afterapplying a skin-pass rolling at a reduction ratio of 1.0%, No. 5 testpieces specified in Japanese Industrial Standard (JIS No. 5 test pieces)were cut out, and the bake hardenability and the yield point elongationafter an artificial aging treatment were measured. The structure of thesteel sheets thus produced and their mechanical properties are shown inTable 2. As is clear from the table, when steels having the chemicalcompositions according to the present invention were hot-rolled underappropriate conditions, both high bake hardenability and anti agingproperty at room temperature were obtained at the same time. TABLE 1steel C Si Mn P S Al Cr Mo V Ca O Ti Nb B N others A 0.0009 0.11 0.080.012 0.007 0.003 0.93 — — 0.002 0.004 — — — 0.0019 — B 0.0011 0.01 0.160.007 0.005 0.035 0.56 0.06 0.01 — <0.001 — — — 0.0052 — C 0.0013 0.070.95 0.010 0.004 0.012 — 0.12 0.02 — 0.003 0.011 0.007 — 0.0102 — D0.0010 0.02 1.15 0.038 0.002 0.010 0.45 — — — 0.002 — — 0.003 0.0061 Sn= 0.03, Cu = 0.1, Ni = 0.05 E 0.0014 0.54 1.56 0.072 0.008 0.002 0.38 —0.01 — 0.002 — — — 0.0015 — F 0.0012 0.02 0.12 0.006 0.009 0.044 — — — —0.001 0.053 0.005 0.004 0.0022 — G 0.0034 0.01 0.10 0.009 0.006 0.039 —— 0.02 — 0.002 0.007 0.005 — 0.0018 — H 0.0012 0.08 0.25 0.005 0.0110.002 0.06 — — — 0.004 — — — 0.0015 — I 0.0008 0.20 0.13 0.064 0.0050.001 0.03 0.01 — 0.003 0.005 — — — 0.0023 — J 0.019 0.01 0.09 0.0080.003 0.016 0.17 — 0.02 — 0.002 — — — 0.0040 — K 0.018 0.02 0.11 0.0070.004 0.015 — — — — <0.001 — — — 0.0045 Sn = 0.05 L 0.045 0.12 0.250.008 0.006 0.004 1.04 0.06 — — 0.002 — — — 0.0053 — M 0.052 1.21 1.240.011 0.001 0.010 0.51 — 0.01 0.002 0.004 — — — 0.0074 — N 0.095 1.171.54 0.003 0.002 0.015 0.66 0.09 0.01 0.003 0.003 — 0.020 — 0.0096 — O0.156 1.94 1.54 0.004 0.001 0.002 0.85 — 0.01 — 0.002 — — — 0.0024 — P0.153 1.99 1.52 0.003 0.001 0.003 0.03 — — 0.002 0.002 — — — 0.0023 —Cr + 3.5 steel Mo + 39 V 0.52 Al/N Remark A 0.93 0.82 Hot rolled,Cold-rolled inventive steel B 1.16  3.50 Hot rolled inventive steel C1.20  0.61 Hot rolled, Cold-rolled inventive steel D 0.45  0.85 Hotrolled, Cold-rolled inventive steel E 0.77  0.69 Hot rolled, Cold-rolledinventive steel F — 10.40 comparative steel G 0.78 11.27 comparativesteel H 0.06  0.69 comparative steel I 0.07  0.23 comparative steel J0.95  2.08 Hot rolled inventive steel K —  1.73 comparative steel L 1.25 0.39 Hot rolled, Cold-rolled inventive steel M 0.90  0.70 Hot rolled,Cold-rolled inventive steel N 1.37  0.81 Hot rolled, Cold-rolledinventive steel O 1.24  0.43 Hot rolled, Cold-rolled inventive steel P0.03  0.68 comparative steel

[0083] TABLE 2 Finishing Average Coiling Temper- cooling temper- BR- BH-BH- ature rate ature Solute TS, YS, EI, 170 160 150 steel ° C. ° C./s °C. Structure N % MPa MPa % MPa MPa MPa *1 *2 Remark A 919 50 550 Singlephase of Ferrite 0.0012 288 157 51 78 72 72 0.06 0.04 according topresent invention A 925  6 550 Single phase of Ferrite 0.0011 291 162 4982 75 73 0.87 0.79 outside present invention B 930 35 450 Single phaseof Ferrite 0.0028 305 175 47 103  100  96 0.11 0.05 according to presentinvention B 923  7 450 Single phase of Ferrite 0.0003 314 183 46 19 13 9 0   0 outside present invention B 934 30 730 Single phase of Ferrite0.0001 313 182 45  2  0  0 0   0 outside present invention C 930 55 400Ferrite + 95% bainitic ferrite 0.0068 376 238 42 119  112  110  0.390.28 according to present invention D 902 35 500 Single phase ofbainitic ferrite 0.0047 423 285 38 108  107  108  0.35 0.26 according topresent invention E 891 30 200 Ferrite + 82% bainitic ferrite 0.0014 466301 35 86 84 84 0.11 0.04 according to present invention E 888  7 450Single phase of Ferrite 0.0012 449 280 37 92 85 81 1.86 1.83 outsidepresent invention F 932 40 500 Single phase of Ferrite 0.0000 295 154 53 5  2  1 0   0 outside present invention G 922 40 730 Single phase ofFerrite 0.0000 292 160 52 58 42 29 0.65 0.65 outside present invention H930 20 500 Single phase of Ferrite 0.0011 286 149 54 65 57 55 0.88 0.81outside present invention I 938 50 400 Single phase of Ferrite 0.0018357 196 44 90 80 78 2.14 2.04 outside present invention J 931 30 500Ferrite + cementite 0.0009 290 175 53 61 55 55 0   0 according topresent invention K 929 30 500 Ferrite + cementite 0.0011 298 180 52 6453 49 0.77 0.78 outside present invention L 906 40 550 Ferrite +cementite 0.0034 341 209 44 107  103  105  0.37 0.32 according topresent invention M 914 21 150 Ferrite + 12% martensite + 0.0022 609 34632 125  120  118  0.08 0 .05 according to Note 1) 1% bainite presentinvention N 890 25 420 Ferrite + 7% austenite + 0.0017 614 413 37 90 9087 0.19 0.14 according to Note 2) 10% bainite present invention O 860 22430 Ferrite + 12% austenite + 0.0010 835 502 32 94 89 87 0.11 0.07according to Note 3) 11% bainite + 1% martensite present invention P 86022 430 Ferrite + 10% austenite + 0.0007 840 520 31 76 71 72 0.94 0.92outside Note 3) 13% bainite present invention

Example 2

[0084] Steels A, C, D, E, F, I, N, O and P among the steels listed inTable 1 were subjected to the following sequential processes: hotrolling at a slab heating temperature of 1,250° C., a finishingtemperature of 930° C. and a coiling temperature of 650° C. to producehot bands 4.0 mm in thickness; pickling; cold rolling at a reductionratio of 80% to produce cold-rolled sheets 0.8 mm in thickness;annealing at a heating rate of 10° C./sec. and the maximum heatingtemperature of 800° C. using a continuous annealing apparatus; coolingat the cooling rates listed in Table 3; averaging treatment for 300 sec.(constant) at different temperatures; and skin-pass rolling at areduction ratio of 1.0% Then JIS No. 5 test pieces were cut out, and thebake hardenability and the yield point elongation after an artificialaging treatment were measured.

[0085] The results are shown in Table 3. As is clear from the table,when steels having the chemical compositions according to the presentinvention were annealed under appropriate conditions, both high bakehardenability and anti aging property at room temperature were obtainedat the same time. TABLE 3 Average cooling Overaging rate temperatureSolute TS, YS, EI, BH170 BH160 BH150 steel ° C./s ° C. structure N % MPaMPa % MPa MPa MPa *1 *2 Remark A 70 250 Single phase of ferrite 0.0010290 151 52 69 66 64 0.05 0.02 according to present invention A  5 250Single phase of ferrite 0.0008 285 146 53 59 54 50 0.67 0.55 outsidepresent invention C 50 150 Single phase of ferrite 0.0046 369 222 41110  108  102  0.35 0.29 according to present invention C  5 150 Singlephase of ferrite 0.0042 370 219 42 115  107  104  2.76 2.44 outsidepresent invention D 60 200 Single phase of ferrite 0.0011 376 233 40 7474 74 0.07 0.02 according to present invention E 50 Not applied Singlephase of ferrite 0.0007 454 265 35 63 60 60 0.01 0.00 according topresent invention F 50 200 Single phase of ferrite 0.0000 288 158 54  1 0  0 0.00 0.00 outside present invention I 40 250 Single phase offerrite 0.0017 354 192 45 84 75 69 2.56 2.23 outside present invention N15 Note 1) 350 Ferrite + 8% austenite + 0.0015 628 425 38 84 82 81 0.120.07 according to 9% bainite present invention O 27 Note 2) 340Ferrite + 12% austenite + 0.0015 820 487 33 89 89 88 0.09 0.05 accordingto 10% bainite present invention P 27 Note 2) 340 Ferrite + 12%austenite + 0.0007 822 497 32 77 72 67 1.06 1.22 outside 10% bainitepresent invention

Example 3

[0086] Steels A and D among the steels listed in Table 1 were subjectedto the following sequential processes: hot rolling at a slab heatingtemperature of 1,250° C., a finishing temperature of 930° C. and acoiling temperature of 650° C. to produce hot bands 4.0 mm in thickness;pickling; cold rolling at a reduction ratio of 80% to producecold-rolled sheets 0.8 mm in thickness; then, using a continuous hot dipgalvanizing line, annealing at a heating rate of 10° C./sec. and amaximum heating temperature of 800° C., cooling at the cooling rateslisted in Table 4, hot dip galvanizing in a zinc bath of 460° C.,reheating at a heating rate of 15° C./sec. to 500° C. and holding at thetemperature for 15 sec; and skin-pass rolling at a reduction ratio of0.8%. Then JIS No. 5 test pieces were cut out, and A1, bakehardenability and the yield point elongation after an artificial agingtreatment were measured.

[0087] The results are shown in Table 4. As is clear from the table,when steel sheets were produced under appropriate conditions, both highbake hardenability and anti aging property at room temperature wereobtained at the same time. TABLE 4 Average Cooling rate Solute TS, YS,EI, BH170 BH160 BH150 Steel ° C./s Structure N % MPa MPa % MPa MPa MPa*1 *2 Remark A 50 Single phase of Ferrite 0.0010 300 151 52 70 70 670.06 0.02 according to present invention A 15 Single phase of Ferrite0.0009 296 146 53 65 63 61 0.12 0.02 according to present invention A  5Single phase of Ferrite 0.0008 295 222 41 59 57 53 1.54 1.37 outside ofpresent invention D 50 Single phase of Ferrite 0.0014 378 240 40 80 8180 0.13 0.08 according to present invention D 15 Single phase of Ferrite0.0014 372 233 40 79 76 75 0.14 0.08 according to present invention D  5Single phase of Ferrite 0.0010 369 230 41 68 65 61 0.88 0.84 outside ofpresent invention

[0088] Industrial Applicability

[0089] A cold-rolled steel sheet, a hot-rolled steel sheet and agalvanized steel sheet having both good bake hardenability and antiaging property at room temperature and capable of maintaining sufficientamount of bake hardenability even when the temperature of BH is low canbe obtained by applying the present invention.

[0090] Since the steel sheet according to the present invention is asteel sheet having painting bake hardenability, when it is used, itsthickness can be made smaller than conventional steel sheets, whichmeans that the weight of the products using the steel sheet can bereduced. The present invention is, therefore, considered to contributeto the conservation of the global environment.

[0091] Moreover, the steel sheet according to the present invention isexcellent also in the collision energy absorption property and,consequently, contributes to enhancing the safety of a car.

1. A cold-rolled steel sheet excellent in painting bake hardenabilityand anti aging property at room temperature characterized by:containing, in mass, 0.0001 to 0.20% of C, 2.0% or less of Si, 3.0% orless of Mn, 0.15% or less of P, 0.015% or less of S and, in addition,0.10% or less of Al and 0.001 to 0.10% of N so as to satisfy theexpression 0.52A1/N<5 and, further, one or more of 2.5% or less of Cr,1.0% or less of MO and 0.1% or less of V so as to satisfy the expression(Cr+3.5Mo+39V) ≧0.1, with the balance consisting of Fe and unavoidableimpurities; having the value of BH170, which is evaluated after applyinga 2% tensile deformation and then a heat treatment at 170° C. for 20min., equal to or more than 45 MPa, and any of the value of BH160, whichis evaluated after applying a 2% tensile deformation and then a heattreatment at 160° C. for 10 min., and the value of BH150, which isevaluated after applying a 2% tensile deformation and then a heattreatment at 150° C. for 10 min., equal to or more than 35 MPa; andhaving the yield point elongation at a tensile test after applying aheat treatment at 100° C. for 1 h. equal to or less than 0.6%.
 2. Ahot-rolled steel sheet excellent in painting bake hardenability and antiaging property at room temperature characterized by: containing, inmass, 0.0001 to 0.20% of C, 2.0% or less of Si, 3.0% or less of Mn,0.15% or less of P, 0.015% or less of S and, in addition, 0.20% or lessof Al and 0.001 to 0.10% of N so as to satisfy the expression0.52A1/N<10 and, further, one or more of 2.5% or less of Cr, 1.0% orless of Mo and 0.1% or less of V so as to satisfy the expression(Cr+3.5Mo+39V) ≧0.1, with the balance consisting of Fe and unavoidableimpurities; having the value of BH170, which is evaluated after applyinga 2% tensile deformation and then a heat treatment at 170° C. for 20min., equal to or more than 45 MPa, and any of the value of BH160, whichis evaluated after applying a 2% tensile deformation and then a heattreatment at 160° C. for 10 min., and the value of BH150, which isevaluated after applying a 2% tensile deformation and then a heattreatment at 150° C. for 10 min., equal to or more than 35 MPa; andhaving the yield point elongation at a tensile test after applying aheat treatment at 100° C. for 1 h. equal to or less than 0.6%.
 3. Acold-rolled or hot-rolled steel sheet excellent in painting bakehardenability and anti aging property at room temperature according toclaim 1 or 2, characterized by containing 0.0005 to 0.004%, in mass, ofsolute N.
 4. A cold-rolled or hot-roiled steel sheet excellent inpainting bake hardenability and anti aging property at room temperatureaccording to any one of claims 1 to 3, characterized by containing,further, 0.0005 to 0.01%, in mass, of Ca.
 5. A cold-rolled or hot-rolledsteel sheet excellent in painting bake hardenability and anti agingproperty at room temperature according to any one of claims 1 to 4,characterized by containing, yet further, 0.0001 to 0.001%, in mass, ofB.
 6. A cold-rolled or hot-rolled steel sheet excellent in painting bakehardenability and anti aging property at room temperature according toany one of claims 1 to 5, characterized by containing, in addition,0.001 to 0.03%, in mass, of Nb.
 7. A cold-rolled or hot-rolled steelsheet excellent in painting bake hardenability and anti aging propertyat room temperature according to any one of claims 1 to 6, characterizedby containing, moreover, 0.0001 to 0.10%, in mass, of Ti so as tosatisfy the expression (N−0.29Ti) >0.0005.
 8. A cold-rolled orhot-rolled steel sheet excellent in painting bake hardenability and antiaging property at room temperature according to any one of claims 1 to7, characterized by containing, furthermore, one or more of Sn, Cu, Ni,Co, Zn, W, Zr and Mg to a total of 0.001 to 1.0%, in mass.
 9. Agalvanized cold-rolled steel sheet excellent in painting bakehardenability and anti aging property at room temperature characterizedby applying a hot dip galvanizing, an alloying hot dip galvanizing or anelectrogalvanizing to a cold-rolled steel sheet according to any one ofclaims 1 and 3 to
 8. 10. A galvanized hot-rolled steel sheet excellentin painting bake hardenability and anti aging property at roomtemperature characterized by applying a hot dip galvanizing, an alloyinghot dip galvanizing or an electrogalvanizing to a hot-rolled steel sheetaccording to any one of claims 2 and 3 to
 8. 11. A method of producing acold-rolled steel sheet excellent in painting bake hardenability andanti aging property at room temperature characterized by: hot-rolling aslab having the chemical composition according to any one of claims 1and 3 to 8 at a temperature 100° C. below the Ar₃ transformationtemperature or higher; cold-rolling the hot-rolled steel sheet thusproduced at a reduction ratio of 95% or less; annealing the cold-rolledsteel sheet thus produced so that the maximum heating temperatureattains the temperature range from 600° C. to 1,100° C.; and thencooling it from the annealing temperature to a temperature of 400° C. orlower at an average cooling rate of 10° C./sec. or more.
 12. A method ofproducing a cold-rolled steel sheet excellent in painting bakehardenability and anti aging property at room temperature characterizedby: hot-rolling a slab having the chemical composition according to anyone of claims 1 and 3 to 8 at a temperature 100° C. below the Ar₃transformation temperature or higher; cold-rolling the hot-rolled steelsheet thus produced at a reduction ratio of 95% or less; annealing thecold-rolled steel sheet thus produced so that the maximum heatingtemperature attains the temperature range from 600 to 1,100° C.; thencooling it from the annealing temperature to a temperature of 400° C. orlower at an average cooling rate of 10° C./sec. or more; and thenapplying to it an averaging treatment at the temperature range from 150to 400° C. for 120 sec. or longer.
 13. A method of producing ahot-dip-galvanized cold-rolled steel sheet excellent in painting bakehardenability and anti aging property at room temperature characterizedby: hot-rolling a slab having the chemical composition according to anyone of claims 1 and 3 to 8 at a temperature 100° C. below the Ar₃transformation temperature or higher; cold-rolling the hot-rolled steelsheet thus produced at a reduction ratio of 95% or less; and then, in acontinuous hot dip galvanizing line, annealing the cold-rolled steelsheet thus produced so that the maximum heating temperature attains thetemperature range from 600° C. to 1,100° C.; then cooling it from theannealing temperature to the temperature of the galvanizing bath at anaverage cooling rate of 10° C./sec. or more, and applying a hot dipgalvanizing to it.
 14. A method of producing analloying-hot-dip-galvanized cold-rolled steel sheet excellent inpainting bake hardenability and anti aging property at room temperaturecharacterized by conducting a heat treatment in a temperature range from460 to 650° C. for 3 sec. or longer after the hot dip galvanizingspecified in the method of producing a hot-dip-galvanized cold-rolledsteel sheet according to claim
 13. 15. A method of producing ahot-rolled steel sheet excellent in painting bake hardenability and antiaging property at room temperature characterized by: hot-rolling a slabhaving the chemical composition according to any one of claims 2 and 3to 8 at a temperature 100° C. below the Ar₃ transformation temperatureor higher; cooling the hot-rolled steel sheet thus produced from the hotrolling finishing temperature to a temperature of 600° C. or below at anaverage cooling rate of 10° C./sec. or more; and then coiling it at atemperature of 550° C. or below.